CAUSES OF DISEASE. 15 



tivity of man in the forest. Insects destroy the foliage, 

 fruit, bark, or wood. Porcupines, deer, cattle, sheep, 

 and other mammals partly live off the green bark, 

 partly browse off the foliage. Eodents sometimes in- 

 jure the roots, particularly of very young trees. 



Lightning will not in every case kill the tree it 

 strikes. As a rule, however, its effect is only realized 

 when a heavy discharge strikes a tree and injures it 

 very severely. But when, as in most cases, the elec- 

 tricity discharged over a forest is divided into a num- 

 ber of weaker currents, which strike as many dominant 

 trees, each of which acts as a conductor, the effect on 

 the single tree becomes less apparent and is easily over- 

 looked. Lightning is very often the cause of spike tops 

 (in case of white firs see also p. 57). Grooves and 

 streaks on the bark and smaller wounds caused by 

 lightning are very common. 



The effects of fire are too well known to need de- 

 scription. It should be remembered ; however, that 

 often a tree may appear green and healthy for as long 

 as two or three years after a fire, only to succumb at 

 last to the injuries received. Decay in very many cases 

 is directly traceable to old fire scars; in fact, fire as a 

 cause of wounds is responsible for more cases of heart - 

 rot than all other factors taken together. The charred 

 surface of exposed wood in the beginning affords pro- 

 tection to the wood underneath it, but soon the charcoal 

 is partly worn off, and the sapwood, which is no longer 

 protected by the bark, dies and checks, thus offering 

 an easy entrance to the germinating spores of wood- 

 destroying fungi. 



